1. Field of the Invention
The present invention relates generally to a line pressure control arrangement for automatic automotive transmissions and more specifically to a line pressure control arrangement which enables the appropriate control of line pressure during a plurality of different shifting operations.
2. Description of the Prior Art
An automatic transmission Service Manual (publication A261CO7) issued in Mar. 1987 relating to the so called RE4RO1A type transmission describes an arrangement which utilizes tabled data for controlling the line pressure during shifting and non-shifting modes of transmission operation. In this arrangement the engine throttle valve opening degree is used to determine the duty cycle of a signal used to control a line pressure control solenoid valve.
However, with this arrangement, because the line pressure is controlled by a single throttle valve position (engine load). Line pressure schedule during shifting operations, it has been found very difficult to successfully control the line pressuring during all of the various shift possibilities. For example, the line pressure level required during a power ON shift is greater than in the case of a corresponding power OFF shift. Further, if the shift occurs at a high vehicle speed the required level of line pressure is higher than in the case the same or similar shift occurs at low speeds even though the position of the throttle valve may be the same.
In addition to this, as this control schedule is not vehicle speed responsive it cannot appropriately regulate the line pressure during shifts which are induced in response to the manual manipulation of the vehicle shift lever. Viz., when a driver manually operates the vehicle shift lever in a manner which forces a downshift outside of the normal load-vehicle speed shift control schedule.
A further problem has been encountered with this type of arrangement in that, when the vehicle is coasting with the throttle valve fully closed, and the transmission undergoes a power OFF 1-2 upshift, a one-way clutch which is included in the transmission gear train, assumes a released position and idles while the engagement level is low. At the same time an accumulator, which is designed to attenuate shift shock at higher load maintains the line pressure which is supplied to a selected friction element (namely the second speed apply chamber 2A of a band brake servo), at a reduced level during this particular shift. This induces the situation wherein, as the line pressure is induced to assume a low level during coasting it does not undergo a timely sharp increase as the pressure reducing function of the accumulator termination is delayed leads to the situation wherein the friction element which is operated by this servo is not strongly engaged and can occur excessive slippage which deteriorates the shift feeling.